The present invention relates to an optical digital regenerator, and more specifically, to an apparatus for regenerating an optical signal in an intact optical state.
As an optical cross-connect node on a large capacity wavelength-division multiplexing optical network in the future, due to the increase of a bit rate per wavelength as well as the increase of the number of multiplexed wavelengths, an optical digital regenerating system for regenerating information as intact optical signals has been widely noticed instead of a system in which optical signals are electrically terminated per wavelength and regeneratively repeated.
In a WDM optical network in which an usable wavelength is assigned to each line in advance, it is necessary that an input wavelength and an output wavelength of an optical digital regenerator are the same. However, in a WDM optical network that positively adopts the wavelength conversion, reusing the wavelengths can reduce the number of wavelengths. In this case, an input wavelength and an output wavelength of an optical digital regenerator are not necessarily the same. Accordingly, conventional optical digital regenerators have employed a structure that comprises two-step wavelength converting parts so as to be widely applicable to any optical network.
A schematic block diagram of a conventional optical digital regenerator is shown in FIG. 4. A signal light from a trunk line system enters an input terminal 10. The signal light (wavelength xcexi) having input the input terminal 10 is divided by an optical coupler 12 and enters a high-speed photodiode 14. The photodiode 14 converts the signal light into an electric signal and applies it to a clock extracting circuit 16. The clock extracting circuit 16 extracts a clock component of the signal light from the output of the photodiode 14.
The input signal light (wavelength xcexi) of the input terminal 10 also enters a wavelength converter 18. The wavelength converter 18 converts the signal light input from the input terminal 10 into another wavelength xcexj. The signal light whose wavelength have been converted into xcexj by the wavelength converter 18 inputs a second wavelength converter 20. The wavelength converter 20 comprises a clock input terminal besides an optical input terminal to which the output light of the wavelength converter 18 enters. The clock extracted by the clock extracting circuit 16 inputs the clock input terminal after being amplified by an amplifier 20 and phase-shifted (adjusted) by a phase shifter 24. The wavelength converter 20 superimposes the signal light of the wavelength xcexj from the wavelength converter 18 on a waveform of an RZ probe pulsed light of wavelength xcexk formed from the clock input through the clock input terminal. By this operation, the wavelength of the signal light is converted from the wavelength xcexj into the wavelength xcexk and at the same time the signal light is retimed and waveform-reshaped.
There are conventional structures in which retiming and waveform-reshaping is performed at a first wavelength converter using extracted clocks (for example, see B. Lavigne et al. xe2x80x98Experimental analysis of SOA-based 2R and 3R optical regeneration for future WDM networksxe2x80x99, OFC ""98, Technical Digest, pp. 324-325, which was published at The Optical Fiber Conference held in February in 1998.). FIG. 5 shows a schematic block diagram of the conventional embodiment.
An input signal light (NRZ optical pulse) of wavelength xcex0 from a trunk line system enters an input terminal 110. The signal light (wavelength xcex0) entered the input terminal 110 is divided by an optical coupler 112 and then inputs a clock regenerating circuit 114. The clock regenerating circuit 114, which comprises a high-speed photodiode, converts the optical pulse from the optical coupler 112 into an electric signal and electrically extracts a clock component contained in the signal light. An LD driving circuit 116 pulse-drives DFB lasers 118 and 120 in accordance with the clock regenerated by the clock regenerating circuit 114. The DFB lasers 118 and 120 respectively laser-oscillate at mutually different wavelengths xcex1 and xcex2 and output pulse lights (probe pulse lights) locked with the regenerated clock from the clock regenerating circuit 114. The probe pulse lights from the DFB lasers 118 and 120 are combined by a multiplexer 122 and then input one facet of a semiconductor optical amplifier (SOA) 124.
The signal light (wavelength xcex0) having input the input terminal 110 also enters a port A of an optical circulator 126 and outputs from its port B. The input signal light from the port B enters the other facet of the SOA 124. While the signal light of the wavelength xcex0 and the probe pulse lights of the wavelengths xcex1 and xcex2 propagate mutually in the opposite directions in the SOA 124, a pulse waveform or bit information of the signal light is copied to the probe pulsed light due to the cross gain modulation effect. That is, the probe pulse light being output from the SOA 124 toward the port B of the optical circulator 126 becomes an RZ pulse conveying the same bit information with the input signal light (wavelength xcex0) of the input terminal 110.
The probe pulse light (wavelength xcex1+xcex2) having output from the SOA 124 enters the port B of the optical circulator 126 and outputs from its port C to be divided into a wavelength xcex1 component and a wavelength xcex2 component by a wavelength demultiplexing element 128. The respective components of the wavelengths xcex1 and xcex2 demultiplexed by the wavelength demultiplexing element 128 propagate on different optical paths 130a and 130b and then multiplexed by a multiplexer 132. The optical paths 130a and 130b are for example predetermined so that the propagation time of the wavelength xcex2 component is delayed by one half bit period in comparison with that of the wavelength xcex1 component. Therefore, the pulse light becomes an NRZ optical pulse after being multiplexed by the multiplexer 132.
The output light of the multiplexer 132 inputs a first port of a first facet of a Mach-Zehnder interferometer (MZI) type wavelength converter 134 and led to one optical path in the MZI wavelength converter 134. The CW laser light from a DFB laser 136 inputs a second facet of the MZI wavelength converter 134. The oscillating wavelength xcexi of the DFB laser 136 is different from both of the oscillating wavelengths xcex1 and xcex2 of the DFB lasers 118 and 120. The output light of the DFB laser 136 is divided in the MZI wavelength converter 134. The divided lights propagate on the two optical paths and combined together again in the MZI wavelength converter 134. The combined light then outputs from a second port of the first facet of the MZI wavelength converter 134. On the one optical path in the MZI wavelength converter 134, the output light of the multiplexer 132 propagates in the opposite direction. The MZI wavelength converter 134 converts the pulse signal of wavelength xcex1+xcex2 into the wavelength xcexi. The waveforms are respectively reversed in the SOA 124 and the MZI wavelength converter 134 and nonreversing regenerated optical signal waveforms are obtained as final outputs. The purpose of the wavelength conversion in the MZI wavelength converter 134 is mainly to improve the extinction ratio.
An optical band pass filter 138 extracts the wavelength xcexi components alone from the output light of the second port of the first facet of the MZI wavelength converter 134. It is for preventing the mixing of lights of the wavelengths xcex0, xcex1 and xcex2 due to the reflection at each part.
In the conventional structure shown in FIG. 4, the optical coupler 12, the high-speed photodiode 14, and the clock extracting circuit 16 are indispensable for extracting the clock components. When optical digital regeneration is performed for each wavelength, the required number of the optical couplers 12 and the photodiodes 14 to be disposed on the node are equal to the number of the wavelengths resulting the increase in costs and the decrease in reliability.
In the conventional structure shown in FIG. 5, two probe light sources are necessary at the first wavelength conversion and it also reduces the reliability and increases the costs. Also, a wavelength demultiplexing element 128 is needed for dividing two wavelengths xcex1 and xcex2 causing the increase of the costs. Since the oscillating wavelength of DFB lasers 118 and 120 has to coincide with the wavelength dividing characteristics of the wavelength demultiplexing element 128 over a long term, it requires a highly advanced wavelength stabilization technique.
An object of the present invention is to solve the above-mentioned problems and to provide an optical digital regenerator for more reliably regenerating an optical pulse signal in an intact optical state.
Another object of the present invention is to provide an optical digital regenerator that requires no high-speed photodiode contributing to the high reliability and the low costs.
An optical digital regenerator according to the present invention is an optical digital regenerator for regenerating an input signal light in a intact optical state, comprising a first optical operating unit having a first probe light generator for generating a first probe light and a first optical operator for converting the waveform of the first probe light output from the first probe light generator in accordance with an optical intensity waveform of the input signal light; a clock extractor for extracting a clock component of the input signal light from a photocurrent generated from the first optical operator; and a second optical operating unit having a second probe light generator for generating a second probe light being pulsed in accordance with the clock output from the clock extractor and a second optical operator for sampling the second probe pulse light output from the second probe light generator according to the first probe light from the first optical operating unit.
Since the clock of the input signal light is extracted from the first optical operator of the first optical operating unit, high-speed photodiodes for extracting clocks and optical couplers for dividing a signal light become unnecessary. Accordingly, the optical elements can be reduced and, as a result, the reliability improves and the costs reduce. In the second optical operating unit, the signal light can get retiming and reshaping by operating the waveform of the probe pulse light formed from the extracted clock with the waveform of the output light of the first optical operating unit.
Assuming that the probe light generated from the first probe light generator is a CW light, the clock extraction becomes easier. The first optical operator comprises for example an EA modulator applied by a predetermined DC bias.
The clock extractor comprises a phase adjuster for adjusting the phase of the extracted clock. Therefore, the retiming and reshaping can be performed in appropriate timing at the optical operation, namely the wavelength conversion in the second optical operating unit.
On the assumption that a first extractor is disposed in the first optical operating unit, which first extractor extracts the probe light operated by the first optical operator and a second extractor is disposed in the second optical operating unit, which second extractor extracts the probe light operated by the second optical operator, even if the signal light and the probe light enter for example from the opposite directions in the respective first and second optical operators, the waveform-operated probe light can be efficiently extracted separately from the signal light. That is, the disposition of respective elements becomes simpler.
When the wavelength xcexj of the probe light generated from the first probe light generator is different from the wavelength xcexi of the input signal light, the problem of interference and crosstalk can be reduced or solved.
When the wavelength xcexk of the probe light generated from the second probe light generator is different from the wavelength xcexj of the probe light generated from the first probe light generator, here again, the problem of interference and crosstalk can be reduced or solved.
By providing an optical pulse stretcher for stretching the pulse width of the optical pulse output from the first optical operating unit, it becomes resistant to the jitter of the input signal light and, thus, the signal light pulse of accurate timing can be obtained in the second optical operating unit. The optical pulse stretcher comprises for example a means for dividing an input light into two and combining them after the divided lights have propagated on different optical paths. The optical pulse stretcher comprises a high dispersion medium. The optical pulse stretcher can also comprises a chirped grating fiber and an optical circulator for supplying the input light to the chirped grating fiber and outputting the reflected light from the chirped grating fiber to the outside. The employment of these passive elements assures the stable operation over a long time and, therefore, a highly reliable 3R regenerator can be realized. In the first optical operating unit, probe light generators for two wavelengths are unnecessary. This also contributes to the improvement of reliability.
The size of the optical pulse stretcher using the high dispersion medium or the chirped fiber grating can be reduced by providing a phase modulator in the first optical operating unit for modulating the phase of the first probe light output from the first probe light generator in accordance with the clock extracted by the clock extractor. The high dispersion medium comprises for example a high dispersion fiber and its length can be reduced extremely, that is, by half.
When the above optical pulse stretcher is employed, it is possible to prevent a bad influence of interference and to improve the stretching effect of the optical pulse by employing at least one of an incoherent light generator and a multi-wavelength light generator as the first probe light generator.
The optical pulse stretcher also can comprise media having different propagation characteristics in mutually orthogonal polarization directions. Although the optical path between the first probe light generator and the optical pulse stretcher need to be a polarization preserving type, the optical pulse can be stretched with a very simple structure.
Furthermore, by providing an optical gate apparatus, between the output of the first optical operating unit and the input of the second optical operating unit, for optically gating the output light of the first optical operating unit in accordance with the clock output from the clock extractor, the extinction ratio can be improved. Using the optical gate apparatus and the above-described optical pulse stretcher together, the probability of failure at sampling in the second optical operating unit is reduced.
When the optical gate apparatus comprises a phase controller for automatically adjusting the phase of the clock output from the clock extractor, the optical gate function of the optical gate apparatus can be followed by the jitter of the input signal light. This operation increases the resistance to the jitter.